For example, there has been proposed a ceramic package sealing structure in which a wide metallization layer is formed on a rectangular-frame-shaped front surface surrounding the opening of a cavity of a ceramic package and which, by means of a seal ring (metal frame) being brazed on an elongated protrusion and on a brazing material charged in a brazing material pool formed on the upper surface of the metallization layer, exhibits improved brazing strength of the brazing material even though a peripheral portion of a metal lid (cap) is welded onto the ring (refer to, for example, Patent Document 1).
Also, there has been proposed a semiconductor device accommodation package in which, by means of a metallization layer having a thickness of 25 μm or more being formed on a rectangular-frame-shaped front surface of a box-like electrically insulating substrate made of ceramic or the like, the front surface surrounding the opening of a concave portion (cavity), the metallization layer absorbs thermal stress generated as a result of seam-welding a metal lid onto a metal frame brazed on the upper surface of the metallization layer, thereby preventing separation of the metallization layer from the front surface of the electrically insulating substrate and enhancing the reliability of a seal for airtightness in the concave portion where a semiconductor device is mounted (refer to, for example, Patent Document 2).
However, in the case of the ceramic package sealing structure described in Patent Document 1, in contrast to improvement in brazing strength of the metal frame, when the metal lid placed on the metal frame is seam-welded for sealing the cavity, generation of resistance heat resulting from the seam welding causes generation of stress stemming from the difference in thermal expansion coefficient between the metallization layer and ceramic used to form the package body, and the stress acts on the metallization layer; as a result, the metallization layer is apt to be separated from the front surface of the package. Thus, there has arisen a problem of a failure to provide a seal by the metal lid or a failure to maintain an airtight seal for a long period of time.
Also, the semiconductor device accommodation package described in Patent Document 2 has involved the following problem: in many cases, the thick metallization layer fails to sufficiently absorb thermal stress generated as a result of seam-welding the metal lid onto the metal frame; additionally, difficulty is encountered in reducing the size of the entire package.
Furthermore, in order to solve the above problems, the following method has been proposed: in order to strongly coat the surface of ceramic of the package or the like with the metallization layer, a fine ceramic powder having a large reaction surface area and a high melting point is used as an additive of a metal paste, and the metal paste is applied onto the surface of alumina ceramic and undergoes baking so as to form a metal coating having strong adhesion on the alumina ceramic (refer to, for example, Patent Document 3).
However, as in the case of the method described in Patent Document 3, when a metal paste which contains ceramic powder is applied onto ceramic and then undergoes baking, in some cases, a glass component contained as an additive in the metal paste is deposit substantially in a planar form on the surface of the metallization layer after baking; i.e., so-called “glass surfacing” arises. As a result, the following problem has been involved: adhesion strength between the metallization layer and a plating layer formed on the surface of the metallization layer drops, resulting in occurrence of a defect, such as separation of plating or plating blister; therefore, when the metal lid and the metal frame are brazed onto the plating layer, joining strength thereof is apt to drop.